The present invention relates to a method and an apparatus for transmitting a packet string having variable packet intervals by converting the packet string to a packet string having even packet intervals with information for reproducing the original packet intervals being attached, more particularly to a method of continuing communications without stopping even when a bit rate becomes higher, that is, when packet intervals become shorter.
When an attempt is made to transmit a packet string in which packets are arranged at variable intervals, that is, a packet string whose bit rate changes, it is indispensable to prepare a data bus provided with a capacity corresponding to the maximum bit rate. Compared with a capacity necessary for transmission at an average bit rate, this makes the bus capacity redundant.
To avoid such a redundancy, a method is proposed in which packets are stored in a buffer (hereinafter referred to as a smoothing buffer) once, then the packets are read out at a fixed rate (hereinafter referred to as a leak rate) and transmitted at a fixed leak rate so as not to require a redundant transmission bus. At this time, however, if packets are transmitted via a smoothing buffer, original packet intervals become unknown. As a result, the original packet intervals cannot be reproduced on the receiving side. To avoid this, interval information is added to each packet so that the original packet intervals can be reproduced in the receiver.
Packet intervals are reproduced as described below. At first, synchronized clocks are started on both transmitting side and receiving side. Then on the transmitting side, the time when the leading edge of a packet is detected at the inlet of the smoothing buffer is stored and a time stamp of a value T decided by adding a specified offset time D to the detected time is attached to the packet before the packet is transmitted. On the receiving side, each received packet is stored in a receiving buffer once and the value T of the time stamp is fetched from the packet. The packet is read out from the receiving buffer when the clock value becomes equal to T to reproduce the original packet interval. FIG. 1A shows a discrete packet string whose average bit rate is A, FIG. 1B shows a discrete packet string to each packet of which a time stamp is attached at the inlet of the smoothing buffer, FIG. C shows a periodical packet string read out from the smoothing buffer and having a leak rate B, and FIG. 1D shows a packet string read out from the receiving buffer and for which the original packet intervals are reproduced.
In the above described method, no problem arises as long as the average bit rate A remained constant. However, when an average bit rate becomes higher, arises the following problems.
If, when the smoothing buffer size is fixed, the bit rate A becomes higher, then the leak rate must be set higher in order to prevent the buffer from overflowing. Thus, the transmission rate must also be raised, and accordingly the total time required for transmitting packets is shortened. Consequently, the above described offset time D must also be set smaller.
If a smaller offset time D is set, however, there is a possibility that the time stamp value calculated by using the smaller offset value becomes less than the value of the time stamp attached to the previous packet. If such an inversion occurs in the time stamp value, the communication is stopped. This is because the time for reproducing the packet attached with the time stamp having the smaller offset value has already lapsed on the receiving side when the previous packet is reproduced.
It is an object of the present invention to solve such problems by providing each packet string having variable packet intervals with information for reproducing original packet intervals so that the packet string can be transmitted continuously, which is converted to a packet string having even packet intervals, even when the bit rate becomes higher.